Nitrogen Fixation HARRY A. CURTIS University of Missouri, Columbia, Missouri FREE NITROGEN AND F M E D NITROGEN
T
HE surface of the earth is a t the bottom of an atmospheric ocean some hundreds of miles deep. This atmosphere is made up almost entirely of a mixture of two gases, oxygen and nitrogen, the latter constituting about four-fifths of the mixture. Inasmuch as life originated and evolved a t the interface between the atmosphere and the earth, it would be strange indeed if living matter did not reveal in its composition something of the environment which fostered it. Actually, we find that oxygen, nitrogen, water, and a score or more of the common elements of the earth's surface are necessary to maintain living organisms. Nitrogen, as an inert component of the earth's atmosphere, was recognized early in the development of chemistry. Its chemical inactivity, as compared with that of other elements, impressed the early chemists. The French chemist Lavoisier called the element "azote," meaning "lifeless." As knowledge of the composition of materials grew, i t must have surprised chemists to find that nitrogen is one of the chemical components of so many things. All living organisms, both animal and plant, were found to contain nitrogen in chemical combinations; ammonia, urea, nitric acid, saltpeter, sal ammoniac, and a large number of other chemicals known to early chemists were eventually found to be nitrogen-containing compounds. All fertile soils were found to contain nitrogen compounds. More and more i t came to be recognized that, although the free nitrogen of the atmosphere is a relatively inert element, nevertheless the element nitrogen, in chemically combined form, plays a most important role both in the animate and the inanimate world. As knowledge of the role of nitrogen grew, it puzzled chemists that there was seemingly no link between the great reservoir of free nitrogen in the air and the chemically combined nitrogen found in all plants and animals. It was shown early in the nineteenth century that plants assimilate nitrogen compounds from the soil, and the value of nitrogenous materials as fertilizers came to be recognized and understood. Nevertheless, the question of the source of the supply of nitrogen compounds in the soil remained unanswered, for the nitrogen content of an unfertilized field did not decrease nearly as rapidly as i t should when nitrogencarrying crops were removed year after year from the field. HOW THE FIXED NITROGEN SUPPLY OF THE SOIL IS MAINTArmED
Liebip, who carried out his mamificent researches in agricultkal chemistry in early ;decades of the nineteenth century, rightly postulated that the nitrogen found in soil could only come from the atmosphere.
Observing that rain water always contains a little ammonia, and knowing that ammonia gas is invariably one of the decomposition products of decaying plants and animals, he advanced an explanation of soil nitrogen supply which, although true, eventually pmved to be only a very small segment of the whole truth. Liebig visualized a great, natural "ammonia cycle," wherein decaying vegetable and animal matter release ammonia to the atmosphere. This ammonia is later dissolved in rain and returned to the soil, there combining with acidic compounds in the soil to form ammonium salts. Plants assimilate nitrogen compounds from the soil. Herbivorous animals obtain their nitrogen from the plants on which they feed, and carnivorous animals obviously secure nitrogen from their nitrogen-carrying foods. But generations of plants and animals die and decay, and the nitrogen of theil bodies returns either to the soil, or, as ammonia, to the atmosphere, again to be recaptured in rain water. So thought Liebig a hundred years ago. The ammonia cycle from the atmosphere to plants, to animals, and thence back to the soil and the atmosphere actually takes place, but the soil is by no means wholly dependent on this cycle for its ever-renewed supply of nitrogen compounds. Long after Liebig's day i t was discovered that there are minute organisms living in the soil which, curiously, are able to assimilate free nitrogen directly from the air and, in their life processes, convert it to nitrogen compounds. These "nitrogenfixing" bacteria are of two sorts, namely those which live free in the soil and those which live only in colonies on the roots of certain plants known as "legumes." Clover, bean, pea, vetch, alfalfa, and lespedesa are among the members of the legume family, and are of great importance in agriculture, not only because these plants are themselves rich in nitrogen, but also because they increase the nitrogen content of the soil in which they grow. In these nitrogen-fixing bacteria we have, indeed, the link between the enormous reservoir of free nitrogen in the atmosphere and all the living plants and animals of the earth. To be sure, man has in recent decades learned how to "fix" the free nitrogen of the air as nitrogen compounds by chemical processes and thus to supplement, in a feeble way, the nitrogen supplied to the soil by the natural processes. How the nitrogen-fixing bacteria fix nitrogen is not known, even today. All the processes that man has invented to fix nitrogen use high temperatures and often high pressures also. The chemistry used by the bacteria is certainly quite different from the drastic sort employed by man.
161
THE GROWWG DEMAND P'OR
COMPOUNDS
Nitrogenous materials were used as fertilizers long
before such materials were recognized as carriers of fixed nitrogen. Likewise, gunpowder, of which potassium nitrate is a component, came into use long before the chemistry of its manufacture or use was uuderstood. The agricultural demand for nitrogenous fertilizers increased as certain areas of the earth became more densely populated, and, in particular, it increased rapidly after the role of nitrogen in agriculture became known about a hundred years ago. Likewise, the demand for gunpowder grew as armies grew in size and number. Almost any nitrogenous material would serve as a fertilizer, but for the manufacture of gunpowder the demand was specifically for potassium nitrate. The earth was ransacked for a supply of potassium u i t r a t m r "saltpeter," as it was called. Only small deposits were found, but about a hundred years ago the natural sodium nitrate deposits in Chile were located. Thereafter "Chilean nitrate" was used in agriculture and eventually a large nitrate industry developed in Chile. The development of a coke industry also made available to agriculture a supplementary supply of nitrogenous fertilizer, for in making coal gas and coke from coal, ammonia is obtained as a co-product, and this ammonia may be converted readily to fertilizer salts. The demand for potassium nitrate for gunpowder was not adequately met until after the middle of the nineteenth century when the potash mines near Stassfurt, Germany, were opened. Then, for the first time, an adequate supply of potassium chloride became available. By reaction between potassium chloride and sodium nitrate, the desired potassium nitrate was readily manufactured. The nineteenth century witnessed not only a rapid expansion in agriculture but also a rapid development of industry. Industrial demands for nitrogen compounds, such as ammonia, nitrates of various kinds, ammonium salts, nitrogen-containing dyes, high explosives, etc., were added to the increasing nitrogen requirements of agriculture and to the military needs. In spite of the new sources of supply, such as the Chilean deposits of sodium nitrate and the coal carbonizing industry, a more or less chronic shortage of nitrogen compounds developed. The amount of by-product ammonia available from the coking of coal was obviously limited by the amount of coke needed in industry. The Chilean nitrate industry became a monopoly, politically controlled by the Chilean government which placed a heavy export duty on the nitrate, and economically controlled by British capital which demanded and obtained an excessively high profit. The monopoly was irksome to all users of nitrate and, as is always the case, created a powerful incentive for the discovery of means of relief. An obvious solution of the problem was to find practical methods of "fixing" the free nitrogen of the atmosphere, i. e., chemical methods by which the free nitrogen could be induced to enter into combination with other elements to form nitrogen compounds.
THE ARC METHOD OR NITROGEN FIXATION
As early as 1772 the English chemist, Priestley, had discovered the "arc method" (electric spark) of fixing nitrogen but he did not understand the chemistry of his discovery. In 1774, Cavendish observed that when an electric arc (spark) was passed through a mixture of five volumes of oxygen mixed with three volumes of air, the gases united to form a gas which was soluble in an alkaline solution. This method of fixing nitfogen was of no commercial importance, however, because electrical energy could be produced a t that time only by prohibitively expensive methods. By the end of the nineteenth century this situation had changed. The transformation of mechanical energy into electrical energy by highly efficient machines had been developed, and hydroelectric plants utilizing the mechanical energy of falling water had been built. The stage was set for a successful arc process of nitrogen hation. Two Americans, Bradley and Lovejoy, built the 6rst commercial plant using the arc method a t Niagara Falls in 1902. The venture was not profitable, but was a distinct advance in the art. A few years later commercially successful plants were built in Norway, using an arc method developed by the physicist Birkeland and the eneineer Evde. of the arc method of nitrogen fixation is relatively simple. The difficulties lie in the technology of putting the chemical reaction to practical use. When a nitrogen-oxygen mixture, such as that which constitutes the atmosphere, is heated, a small portion of the gases unite to form nitric oxide, thus:
he-&emis&
N3fOze2NO
The reversed arrows used in the above expression indicate that the reaction moves to the right or to the left as conditions change. All chemical reactions are actually of this nature, but in many cases the conditions are such as to favor the reaction in one direction so overwhelmingly that chemists ordinarily need not consider both reactions. In the present instance, however, the process can be understood only by considering both the forward and reverse reactions as represented in the equilibrium expression written above. At ordinary temperatures the equilibrium between free nitrogen and free oxygen on the one hand and nitric oxide on the other is such that practically no nitric oxide may be detected in the mixture. At the extremely high temperature of the electric arc, the reaction proceeds a short measure to the right, and a small percentage of nitric oxide appears in the hot mixture of gases. If, now, the gases are allowed to cool slowly, the reaction moves to the left until practically no nitric oxide remains. An arc process may therefore appear to be a useless procedure, for in the end only the original uncombined gas remains. Here is where the trick comes in which makes the arc process practical. The chemical expression written above does not indicate anything as to the rate a t which the reactions may move to right or left. It is well known that most chemical reactions speed up as the temperature of the
reactants is raised, and slow down as the temperature falls. At the very high temperature of the electric arc, one would expect that any reaction which could take place under such conditions would take place quickly. The trick in the arc method is to remove the very hot gas mixture from the arc and to chill the mixture very rapidly by diluting it with a large proportion of air a t ordinary temperature. The sudden cooling slows up the reverse reaction, i. e., the decomposition of the nitric oxide, until the reaction practically ceases and a mixture of air canying one to two per cent of nitric oxide is obtained. The nitric oxide does not remain as such in the mixture, for as the temperature falls below about 600°C. another equilibrium reaction sets in. This reaction may he represented most simply by the expression
chemists more than one hundred years ago. As early as 1842 attempts were made to establish commercial scale nitrogen-fixing plants using the so-called cyanide method. I t had been discovered in 1828 that when nitrogen is brought into contact with a hot mixture of an alkali carbonate and carbon in an iron tube, an alkali cyanide is formed. Early attempts to use this method failed. During World War I, further trials of the method were made, but the small plants that vere built soon were abandoned. THE CYANAMIDE METHOD OF NITROGEN PIXATION
The cyanamide method of nitrogen fixation was discovered by Frank and Caro in the closing decade of the nineteenth century. In studying nitrogen furation by the cyanide method, it was found that nitrogen would 2NO O2 F? 2N02 combine with certain metal carbides to form both In the arc process the cooled gases from the arc are cyanides and cyanamides. With calcium carbide at brought into contact with water in absorption towers. about 100O0C.,the product was almost entirely calcium Here the nitrogen peroxide (NOz) reacts with water, cyanamide. Later it was found that calcium cyanathus: mide may be used directly as a nitrogen-carrying fertilizer. Factories using the new method of nitrogen furation It will be noted that for every three molecules of NO2 were built in Germany, Italy, and elsewhere in the which react with water, one molecule of NO is released. period 1905-10. In 1908 a plant using this method was This NO will then react with the oxygen present in the built by the American Cyanamid Company on the large excess of air in the gas mixture, again forming Canadian side of Niagara Falls. NOz. Unfortunately the reaction: In the cyanamide process, coke and lime are heated in an electric furnace to form calcium carhide, thus: 3C + CaO CaCa + CO moves to the right rather slowly under the conditions which exist in the absorption tower. Because the proThe calcium carbide removed from the electric furportion of NO in the gas mixture coming from the arc nace is cooled, ground, placed in small cylindrical ovens, is so small, and because the reactions in the absorption and treated with nitrogen gas a t a temperature of about tower are so slow, very large and therefore very costly 100O0C. The nitrogen may be obtained by liquefying absorption towers are required. Furthermore, the air at low temperatures and separating the nitrogen efficiency of the arc as a method of fixing nitrogen is from the oxygen. Under the conditions in the ovens, very low. Most of the electrical energy is used to heat the nitrogen reacts slowly with the calcium carbide to the gas mixture in the arc and not to form the small pro- form calcium cyanamide, thus: portion of nitric oxide obtained. Therefore this method CaC, Ne ~2 CaCN, C can be used only where large amounts of electrical energy can be produced economically, and where such The cyanamide mixed with the carbon formed in the energy cannot find more profitable uses. It was for this reaction is removed from the ovens as a solid cake. It reason that the arc process plants were located in Nor- is then ground, oiled to reduce its dustiness (or granuway where hydroelectric energy could be produced lated), and sold as a fertilizer. Its use as a fertilizer is cheaply and where no large industries were located at somewhat handicapped by the fact that it is dusty, even the time the arc plants were built. after oiling or granulating, and it is alkaline and toxic. In the Norwegian arc process plants, the nitric acid It is still manufactured on a fairly large scale and used in from the absorption towers was neutralized with lime agriculture and for other purposes, but it appears probato form calcium nitrate. This "Norwegian nitrate" ble that the cyanamide method will eventually be abanabsorbs moisture readily from moist air and cakes badly. doned as was the arc method of nitrogen fixation. It was therefore packed in tight wooden kegs for the market. It was sold as a fertilizer and was rather TEE DIRECT SYNTHETIC AMMONIA METHOD OF NITROGEN FIXATION widely used for several years. The arc method of fixing nitrogen was applied in a few small plants located elseA sound scientific foundation for the direct synthetic where than in Norway, but was eventually replaced, ammonia process was laid during the f i s t decade of the even in Norway, by more economical methods. present century. Equilibrium reactions, such as discussed in describing the arc method of nitrogen fixation, THE FIXATION OF NITROGEN AS A CYANIDE were coming to be more fully understood. It had long Another method of k i n g nitrogen was familiar to been suspected that an equilibrium existed between
+
+
+
hydrogen and nitrogen on the one hand and ammonia on the other, i. e.
doubt of value in the studies which Professor Haber and his students had undertaken. With the catalysts which had been discovered, and 3Ha Nn 2NHz with an accurate knowledge of the equilibrium involved, Attempts to study this equilibrium did not yield i t began to appear that a practical process of nitrogen much information, however. Hydrogen and nitrogen, fixation might be developed. In 1908 the Badische when brought together under ordinary conditions, ap- Anilin-und Soda Fabrik undertook this task. The parently formed no ammonia. Several investigators technical difficulties involved were obviously great, but had, on the other hand, reported that the decomposition the German company had a t its command great reof ammonia was never quite complete when it was sources in the way of chemical and engineering talent as passed over hot solids or through hot iron tubes. The well as in funds. trace of ammonia remaining indicated that there might The lirst synthetic ammonia plant in the world was indeed be a truly reversible reaction such as represented put into operation a t Oppau, Germany, in 1913. Then by the expression written above. came World War I and the blockade of Germany. A The fact that no ammonia could be detected when stock pile of Chilean nitrate had been accumulated in hydrogen and nitrogen were brought together a t Germany for what was intended to be a short war. ordinary temperatures could be explained on either Cut off from imports of Chilean nitrate, and with no of two assumptions, namely, (1) the equilibrium under end of the war in sight, it was imperative that synthetic ordinary conditions was such that the proportion of nitrogen production in Germany be increased greatly in ammonia was too small to be detected, or, (2) the reac- order to meet the enormous military demand for extion between hydrogen and nitrogen was so slow under plosives, all of which are nitrogen compounds. The ordinary conditions that not enough ammonia was existing cyanamide plants were expanded and a few formed during the experiment to be detected by chemi- small arc plants were set up, but the power reqnirecal methods. That this latter hypothesis was the cor- ments of these processes were so high that they could rect one was indicated by the fact that when ammonia not meet Germany's need for fixed nitrogen. Under was passed through a hot iron tube, enough still re- these conditions, the synthetic ammonia plant a t mained nndecomposed to be detected chemically. Oppau was expanded greatly and eventually a very Finally, early in the present century, the German chem- large synthetic ammonia plant was built near Merseist Fritz Haber and his research students undertook a burg. Thus Germany solved her nitrogen problem, prolonged study of the reaction. and the synthetic ammonia process developed with unThe equilibrium was a difficult one to study. If the usual rapidity. reaction between hydrogen and nitrogen were really After World War I, synthetic ammonia plants were extremely slow a t ordinary temperature, then it was built in many countries. By 1926 the Chilean monopnatural to try measurements a t higher temperatures. oly was completely broken, and the price level of This in itself was difficult but not the only difficulty. nitrogen products now is fixed by the cost of making It happens that higher temperatures favor the right-to- synthetic ammonia. left reaction written above, i. e., the already small All synthetic ammonia plants are alike in utilizing proportion of ammonia present when the two reactions catalysts to establish an equilibrium between hydrogen, are in equilibrium is decreased almost to the vanishing nitrogen, and ammonia. The plants differ greatly in point. High pressure, however, favors the left-to-right other respects. Many catalysts have been discovered; reaction. This is the result of the fact that three mole- the one most commonly used is the magnetic oxide of cules of hydrogen combine with one of nitrogen to form iron (Fes04)into which are incorporated small amounts two of ammonia gas. It is a well-known principle that of other compounds, called "promoters." Alumina equal numbers of molecules of different gases occupy the (AlzOs) and potassium oxide (KzO) are the promoters same v o l u m e s a t the same conditionsand hence the most commonly used. Hydrogen may be prepared in many ways. The ammonia has only half the volume of the hydrogennitrogen mixture. The "squeezing together" effect of original German plant used the water gas method. pressure favors this reduction in volume, and therefore Other plants use coke-oven gas. Some plants use electrolytic hydrogen and some use the gases obtained the formation of ammonia. The technic of working a t high pressures was not from the thermal decomposition (cracking) of oil or highly developed forty years ago. However, Haber natural gas. All plants naturally use nitrogen from the and his students were eventually able to overcome all air. In all cases the hydrogen-nitrogen mixture must the difficulties and to measure with considerable be thoroughly purified, and in most plants the equipaccuracy the proportions of hydrogen, nitrogen, and ment for this step in the process is the major portion of ammonia present under various conditions of tempera- the plant. Various methods are used in purifying the ture and pressure. Catalysts were found which would hydrogen-nitrogen gas mixture and plants differin both accelerate the combination of hydrogen and nitrogen. method and equipment used. Professor Nernst, another German chemist, was a t this As mentioned above, pressure favors the synthesis of time engaged in the study of gaseous reactions under ammonia from nitrogen and hydrogen. Plants have pressure, and the technic which he developed was no been built to operate a t various pressures in the range
+ *
from 100 atmospheres to 900 atmospheres. The present trend appears to be toward use of pressures in the neighborhood of 350 atmospheres (about 5150 lb. per sq. in.). In all plants, the purified mixture of hydrogen and nitrogen is passed through a bed of catalyst material. The proportion of ammonia in the gas issuing
from the catalyst chamber depends on the temperature and pressure maintained in the catalyst chamber. The ammonia in the mixture is removed, usually by refrigeration, and the remaining gases are returned to the system, mixed with additional hydrogen-nitrogen gas, and passed again through the catalyst.
